Method and apparatus for transmitting wideband speech signals

ABSTRACT

A method and an apparatus for transmitting a speech signal are provided. A speech signal transmitter includes a quadrature mirror filter, a bas sub-band encoder, an enhancement sub-band encoder, and a network connector. The quadrature mirror filter receives a speech signal, divides the speech signal into an enhancement band speech signal and a base band speech signal, and outputs the enhancement band speech signal and the base band speech signal. The base sub-band encoder receives and encodes the base band speech signal. The enhancement sub-band encoder receives and encodes the enhancement band speech signal. The network connector multiplexes the encoded enhancement band speech signal and the encoded base band speech signal based on the kinds of networks over which speech signals are transmitted, and transmits the multiplexed signals to the networks. A speech signal is multiplexed and transmitted by various methods based on the kinds of networks. Thus, the speech signal can be efficiently transmitted.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method and an apparatus fortransmitting speech signals, and more particularly, to a method and anapparatus for wideband encoding of speech signals and transmission ofthe encoded bit stream.

[0003] 2. Description of the Related Art

[0004] Conventionally, various coding methods have been proposed todigitize and process speech signals. General speech signal processingmethods are classified into two types: in one, 16 bit linear pulse codemodulation data is obtained by sampling input analog speech signals at 8kHz and input to an encoder; in the other, 16 bit linear pulse codemodulation data is obtained by sampling input analog speech signals at16 kHz and input to an encoder. In the former method, speech signals arecoded by methods including G.711-G.712 pulse code modulation (PCM) andG.720-G.729 non-PCM standards of the International TelecommunicationUnion—Telecommunication Standardization Sector (ITU-T). In the lattermethod, speech signals are coded by G.722 and G.722.1 of the ITU-T andan adaptive multi-rate wideband (AMR-WB) method, which will be used inIMT-2000.

[0005] Here, G.723.1, which is a standard for compressing multimediasignals at a lower rate, is an algorithm for compressing and restoringinput speech at a dual rate of 5.3/6.3 kbit/s and provides toll qualityon a cable network. G.723.1 uses a Hybrid coding technique combining awaveform coding method and a parametric coding method, and is a CodeExcited Linear Prediction (CELP)-based speech coder. G. 729, which is astandard for IMT-2000 to expand the frequency efficiency for mobilecommunications, is an algorithm for compressing and restoring inputspeech at a rate of 8 kbit/s. G.729A is reduced complexity version ofthe G.729 coder. This version is bitstream interoperable with the fullversion.

[0006] G.729A also provides toll quality on a cable network and uses aHybrid coding technique and a CELP-based speech coder. ITU-T G.722,which is a standard for coding wideband audio signals, has atransmission rate of 64, 56, or 48 kbit/s and face-to-face communicationquality. Also, G.722 divides one band into two sub-bands and codes eachof the two sub-bands, using an Adaptive Differential Pulse CodeModulation (AD-PCM) method.

[0007] Methods and apparatuses for providing toll quality on a cablenetwork and coding speech at a lower rate have enabled new services inmobile communications and telephony due to high frequency efficiency. Inparticular, the services using Voice over Internet Protocol (VoIP) overInternet networks are rapidly becoming widespread because of their lowtelephone rates. However, conventional coding methods and apparatuseshave the problem of low service quality due to low toll quality and longdelay over Internet networks. Thus, the conventional coding methods andapparatuses do not have a good reputation.

[0008] Accordingly, in order to solve these problems, various attemptshave been made. For example, if speech signals are sampled at afrequency of 16 kHz in a VoIP system before coding, the quality of thespeech signals may be much improved. However, current 16 kHz widebandspeech codecs are not at all compatible with the codec currently used inthe VoIP service. Thus, a new system is needed. Also, since widebandsignals have a wide frequency bandwidth, networks with a large dataprocessing capacity are needed. Thus, starting new services in disregardof current systems has many difficulties.

SUMMARY OF THE INVENTION

[0009] To solve the above-described problems, it is a first object ofthe present invention to provide a speech signal transmitter which iscompatible with a conventional system and provides high quality widebandspeech signals.

[0010] It is a second object of the present invention to provide aspeech signal receiver which is compatible with a conventional systemand provides high quality wideband speech signals.

[0011] It is a third object of the present invention to provide a methodof transmitting speech signals which is compatible with a conventionalsystem and provides high quality wideband speech signals.

[0012] It is a fourth object of the present invention to provide amethod of receiving speech signals which is compatible with aconventional system and provides high quality wideband speech signals.

[0013] Accordingly, to achieve the first object, there is provided aspeech signal transmitter including a quadrature mirror filter, a basesub-band encoder, an enhancement sub-band encoder, and a networkconnector. The quadrature mirror filter receives a speech signal,divides the speech signal into an enhancement band speech signal and abase band speech signal, and outputs the enhancement band speech signaland the base band speech signal. The base sub-band encoder receives andencodes the base band speech signal. The enhancement sub-band encoderreceives and encodes the enhancement band speech signal. The networkconnector multiplexes the encoded enhancement band speech signal and theencoded base band speech signal based on the kinds of networks overwhich speech signals are transmitted, and transmits the multiplexedsignals to the networks. The base sub-band encoder encodes the base bandspeech signal based on G.723.1 or G.729A. The network connectorsimultaneously packets the enhancement band speech signal and the baseband speech signal or packets only the base band speech signal, if thenetworks are composed only of networks guaranteeing or of networks notguaranteeing quality of service, and packets and transmits the base bandspeech signal to the networks guaranteeing quality of service andpackets and transmits the enhancement band speech signal to the networksnot guaranteeing quality of service, if the networks are composed ofnetworks guaranteeing quality of service and networks not guaranteeingquality of service. The speech signal has a sampling frequency of 16 kHzand a bandwidth of 0-8 kHz, the enhancement band speech signal has asampling frequency of 8 kHz and a bandwidth of 4 kHz-8 kHz, and the baseband speech signal has a sampling frequency of 8 kHz and a bandwidth of0-4 kHz. The speech signal transmitter further includes an acoustic echocanceller for receiving the speech signal, canceling echoes from thespeech signal, and outputting the speech signal to the quadrature mirrorfilter.

[0014] To achieve the second object, there is provided a speech signalreceiver including a network connector, an enhancement sub-band decoder,a base sub-band decoder, and a quadrature mirror filter. The networkconnector receives a speech signal, within which an enhancement bandspeech signal and a base band speech signal are multiplexed based on thekinds of networks through which speech signals are transmitted,demultiplexes the multiplexed speech signal, and extracts an enhancementband digital speech signal and a base band digital speech signal. Theenhancement sub-band decoder receives and decodes the enhancement bandspeech signal. The base sub-band decoder receives and decodes the baseband speech signal. The quadrature mirror filter synthesizes the decodedenhancement band speech signal and the decoded base band speech signalinto a single speech signal. The base sub-band decoder decodes the baseband speech signal based on G.723.1 or G.729A. The enhancement bandspeech signal has a sampling frequency of 8 kHz and a bandwidth of 4-8kHz, and the base band speech signal has a sampling frequency of 8 kHzand a bandwidth of 0-4 kHz. The speech signal receiver further includesan acoustic echo canceller for canceling echoes from the synthesizedsingle speech signal.

[0015] To achieve the third object, there is provided a method oftransmitting a speech signal. An analog speech signal is received andconverted into a digital speech signal. The digital speech signal isdivided into an enhancement band speech signal and a base band speechsignal. The enhancement band speech signal and the base band speechsignal are separately encoded. The encoded enhancement band speechsignal and the encoded base band speech signal are multiplexed based onthe kinds of networks to which the speech signal is transmitted, and themultiplexed enhancement band speech signal and the multiplexed base bandspeech signal are transmitted to the networks. The step of receiving andconverting the analog speech signal includes canceling echoes from thedigital speech signal. In the step of multiplexing the encodedenhancement band speech signal and the encoded base band speech signaland transmitting the multiplexed enhancement band speech signal and themultiplexed base band speech signal to the networks, if the networks arecomposed only of networks guaranteeing or of networks not guaranteeingquality of service, then the enhancement band speech signal and the baseband speech signal are simultaneously packeted or only the base bandspeech signal is packeted, and if the networks are composed of networksguaranteeing quality of service and networks not guaranteeing quality ofservice, the base band speech signal is packeted and transmitted to thenetworks guaranteeing quality of service and the enhancement band speechsignal is packeted and transmitted to the networks not guaranteeingquality of service.

[0016] To achieve the fourth object, there is provided a method ofreceiving a speech signal. A speech signal, within which an enhancementband speech signal and a base band speech signal are multiplexed basedon the kinds of networks through which speech signals are transmitted,is received. The multiplexed enhancement band speech signal and themultiplexed base band speech signal are demultiplexed, and anenhancement band digital speech signal and a base band digital speechsignal are extracted. The enhancement band digital speech signal and thebase band digital speech signal are decoded. The decoded enhancementband digital speech signal and the decoded base band digital speechsignal are synthesized into a single speech signal. The step ofsynthesizing the decoded enhancement band digital speech signal and thedecoded base band digital speech signal into a single speech signalincludes canceling echoes from the synthesized one speech signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The above objects and advantages of the present invention willbecome more apparent by describing in detail preferred embodimentsthereof with reference to the attached drawings in which:

[0018]FIG. 1 is a diagram showing a preferred embodiment of thetransmission of wideband speech signals via a network according to thepresent invention;

[0019]FIG. 2 is a diagram of the structure of a preferred embodiment ofa speech signal coder according to the present invention;

[0020]FIG. 3 is a diagram of the structure of a preferred embodiment ofa speech signal transmitter according to the present invention;

[0021]FIG. 4 is a flowchart of a preferred embodiment of a method oftransmitting speech signals according to the present invention;

[0022]FIG. 5 is a diagram of the structure of a preferred embodiment ofa speech signal decoder according to the present invention;

[0023]FIG. 6 is a diagram of the structure of a preferred embodiment ofa speech signal receiver according to the present invention;

[0024]FIG. 7 is a flowchart of a preferred embodiment of a method ofreceiving speech signals according to the present invention;

[0025]FIG. 8 is a diagram of the structure of a preferred embodiment ofan audio communication terminal according to the present invention; and

[0026]FIG. 9 is a diagram of the structure of a general Voice overInternet Protocol (VoIP) service.

DETAILED DESCRIPTION OF THE INVENTION

[0027] Hereinafter, a preferred embodiment of the present invention willbe described in detail with reference to the attached drawings.

[0028]FIG. 1 is a diagram showing a preferred embodiment of thetransmission of a wideband speech signals via a network according to thepresent invention. Here, a network guaranteeing the quality of service(QoS) and a network not guaranteeing the QoS are simultaneouslyprovided.

[0029] Internet Protocol (IP) terminals 101 and 106, which are terminalssuch as microphones, speakers, and telephones for receiving andoutputting speech signals, generate and transmit IP packets. Theterminals may have various shapes such as telephone sets or personalcomputers (PC). When a packet generated in the IP terminal 101 istransmitted to the IP terminal 106, the packet is divided into a baseband packet 105 and an enhancement band packet 102. The numbers inpackets 102 and 105 represent time stamps. The base band packet 105 hasimportant information on the speech communications and thus istransmitted via a network 104 guaranteeing QoS. If only the base band isused, a person can at least communicate with another person, althoughthe intelligibility and naturalness of their voice is considerablydecreased. Thus, if a user wants face-to-face sound quality, the userdecodes the enhancement band packet 102 transmitted over the network 103not guaranteeing the QoS, giving high sound quality. Since a packettransmitted over the network 103 not guaranteeing the QoS has irregulardelay, packet error, and jitter, the quality of sound is maximized usinga device for improving the QoS such as a jitter buffer and a frameerasure concealment (FEC) device in order to synchronize the packetsreceived by the receiving IP terminal 106.

[0030]FIG. 2 is a diagram of the structure of a preferred embodiment ofa speech signal coder according to the present invention. A quadraturemirror filter (QMF) 201 receives a digital speech signal, which is a 16bit linear pulse code modulation (PCM) signal having a bandwidth of 0-8kHz sampled at 16 kHz, and outputs a base band signal having a bandwidthof 0-4 kHz sampled at 8 kHz and an enhancement band signal having abandwidth of 4-8 kHz sampled at 8 kHz. An enhancement sub-band encoder202 receives and encodes the enhancement band signal using a waveformcoding method or a transform coding method, to output an enhancementspeech signal. A base sub-band encoder 203 receives and encodes the baseband signal according to G.723.1 or G.729A methods, to output a baseband speech signal.

[0031]FIG. 3 is a diagram of the structure of a preferred embodiment ofa speech signal transmitter according to the present invention. An A/Dconverter 301 receives a user's analog speech signal through amicrophone and converts the analog speech signal into a digital speechsignal.

[0032] An acoustic echo canceller 302 cancels echoes from the speechsignal received from the A/D converter 301. The acoustic echo canceller302 may be an acoustic echo canceller according to ITU-T G.167. Thesignals input into and output from the acoustic echo canceller 302 are16 bit linear PCM signals sampled at 16 kHz, with a bandwidth of 0-8kHz.

[0033] An encoder 303 receives the signal output from the acoustic echocanceller 302 and separately encodes a base band signal and anenhancement band signal. The detailed structure of the encoder 303 isshown in FIG. 2.

[0034] An IP Network Interface 304 multiplexes an input signal composedof a base band and an enhancement band in various ways, according to thestructure of the IP Network, to efficiently transmit packets. If all IPnetworks provided for speech signal communication guarantee the QoS,then the base band signal and the enhancement band signal aresimultaneously packeted or only the base band signal is packeted. If anetwork guaranteeing the QoS and a network not guaranteeing the QoS aresimultaneously provided, the relatively important base band signal istransmitted through the network guaranteeing the QoS, and theenhancement band signal is transmitted through the network notguaranteeing the QoS. Here, although packet errors and jitter occur inthe network not guaranteeing the QoS, sound quality is higher than whenonly the base band signal is used. If all the IP networks do notguarantee the QoS, then the base band signal and the enhancement bandsignal are simultaneously packeted or only the base band signal ispacketed.

[0035] The IP Network Interface 304 generates packets in the form shownin FIG. 9. Currently VoIP is composed of the combination of variousprotocols, but the present invention may be applied to the combinationof any protocols mentioned here.

[0036]FIG. 4 is a flowchart of a preferred embodiment of a method oftransmitting speech signals according to the present invention. Ananalog speech signal is received and is converted into a digital speechsignal in step 401. Echoes are cancelled from the digital speech signalin step 402. The digital speech signal is divided into an enhancementband signal and a base band signal in step 403. The enhancement bandsignal and the base band signal are separately encoded in step 404. Theencoded speech signal is multiplexed in various combinations accordingto the kinds of connected networks in step 405. The multiplexed signalis packeted and transmitted to the networks in step 406.

[0037] Here, the digital speech signal, into which the analog speechsignal is converted, is a 16 bit linear PCM signal sampled at 16 kHz.The base band signal is sampled at 8 kHz and has a bandwidth of 0-4 kHz.The enhancement band signal is sampled at 8 kHz and has a bandwidth of4-8 kHz. The base band signal may be coded by G.723.1 or G.729A methods.The enhancement band signal may be coded by a waveform coding method ora transform coding method.

[0038]FIG. 5 is a diagram of the structure of a preferred embodiment ofa speech signal decoder according to the present invention. Anenhancement sub-band decoder 501 receives and decodes a codedenhancement band speech signal using a waveform coding method or atransform coding method, to output an enhancement band speech signal.

[0039] A base sub-band decoder 502 receives and decodes a coded baseband speech signal according to G.723.1 or G.729A methods, to output abase band speech signal.

[0040] A quadrature mirror filter (QMF) 503 receives the enhancementband speech signal and the base band speech signal from the enhancementsub-band decoder 501 and the base sub-band decoder 502. The QMF 503synthesizes the enhancement band speech signal and the base band speechsignal into one speech signal and outputs the one speech signal.

[0041]FIG. 6 is a diagram of the structure of a preferred embodiment ofa speech signal receiver according to the present invention.

[0042] An IP Network Interface 601 receives and demultiplexes amultiplexed base band speech signal and a multiplexed enhancement bandspeech signal, to extract and output a coded enhancement base speechsignal and a coded base band speech signal.

[0043] A decoder 602 receives the extracted enhancement band speechsignal and base band speech signal from the IP Network Interface 601,decodes them, synthesizes them into one speech signal and outputs thesingle speech signal. Here, the decoding is the same as the codingdescribed above. The detailed structure of the decoder 602 was describedin detail with reference to FIG. 5.

[0044] An acoustic echo canceller 603 receives the synthesized speechsignal from the decoder 602 and cancels echoes from the synthesizedspeech signal. The acoustic echo canceller 603 may be an acoustic echocanceller according to ITU-T G.167.

[0045] A D/A converter 604 receives the speech signal from which theechoes are cancelled, and converts the speech signal into an analogsignal, which is provided to a user via an outputting portion.

[0046]FIG. 7 is a flowchart of a preferred embodiment of a method ofreceiving speech signals according to the present invention. Amultiplexed speech signal is received according to the kinds ofnetworks, in step 701. The multiplexed speech signal is demultiplexedand divided into an enhancement band signal and a base band signal, instep 702. The enhancement band signal and the base band signal areseparately decoded in step 703. The decoded enhancement band signal andbase band signal are synthesized into one speech signal, in step 704.Echoes are cancelled from the synthesized speech signal in step 705. Thespeech signal is converted from a digital signal into an analog signalin step 706.

[0047] A method of decoding the speech signal is the same as the methodapplied to the decoder and the speech signal receiver described withreference to FIGS. 5 and 6.

[0048]FIG. 8 is a diagram of the structure of a preferred embodiment ofan audio communication terminal according to the present invention.Referring to FIG. 8, an A/D converter 801 receives a user's voice as ananalog speech signal through a microphone, and converts the analogspeech signal into a digital speech signal.

[0049] An acoustic echo canceller 802 cancels echoes from the digitalspeech signal output from the A/D converter 801. The acoustic echocanceller 802 also receives a digital speech signal from a receivingside QMF 809 and cancels echoes from the digital speech signal.

[0050] A transmitting side QMF 803 receives a speech signal, from whichechoes are cancelled from the user's speech signal, and divides thespeech signal into a base band signal having a bandwidth of 0-4 kHz andan enhancement band signal having a bandwidth of 4-8 kHz.

[0051] An enhancement sub-band encoder 804 receives the enhancement bandsignal from the transmitting side QMF 803, codes the enhancement bandsignal according to a waveform coding method or a transform codingmethod, and outputs the coded enhancement band signal to a networkconnector 806.

[0052] A base sub-band encoder 805 receives the base band signal fromthe transmitting side QMF 803, codes the base band signal according toG.723.1 or G.729A methods, and outputs the coded base band signal to thenetwork connector 806.

[0053] The network connector 806 receives the coded enhancement bandsignal and base band signal from the enhancement sub-band encoder 804and the base sub-band encoder 805, multiplexes the coded enhancementband signal and base band signal, and transmits the multiplexed signalto the networks the enhancement band signal and the base band signal,which are the speech signals, are combined and multiplexed according tothe kinds of networks to which the speech signal is transmitted. Inother words, if all IP networks provided for the communication of speechsignals guarantee the QoS, the enhancement band signal and the base bandsignal are simultaneously packeted or only the base band signal ispacketed. When a network guaranteeing the QoS and a network notguaranteeing the QoS are simultaneously provided, the relativelyimportant base band signal is transmitted to the network guaranteeingthe QoS, and the enhancement band signal is transmitted to the networknot guaranteeing the QoS. Here, although packet errors and jitter occurin the network not guaranteeing the QoS, sound quality is higher thanwhen only the base band signal is used. If all the IP networks do notguarantee the QoS, the base band signal and the enhancement band signalmay be simultaneously packeted or only the base band signal may bepacketed.

[0054] The network connector 806 receives and demultiplexes a signal,from the multiplexed base band signal or the enhancement band signal,based on the kinds of networks, and extracts and outputs a codedenhancement band speech signal and base band speech signal.

[0055] An enhancement sub-band decoder 807 receives the extractedenhancement band speech signal from the network connector 806 anddecodes the extracted enhancement band speech signal according to awaveform coding method or a transform coding method.

[0056] A base sub-band decoder 808 receives the extracted enhancementband speech signal from the network connector 806 and decodes theextracted enhancement band speech signal according to G.723.1 or G.729Amethods.

[0057] A receiving side QMF 809 receives the decoded speech signals fromthe enhancement sub-band decoder 807 and the base sub-band decoder 808,synthesizes the decoded speech signals into one speech signal, andoutputs the single speech signal to the acoustic echo canceller 802.

[0058] A D/A converter 810 receives a speech signal, from which echoesare cancelled, from the acoustic echo canceller 802 and converts thespeech signal into an analog speech signal. The analog speech signal maybe provided to a user via a speech signal outputting device.

[0059] Here, a digital speech signal, into which an analog speech signalis converted, is a 16 bit linear PCM signal sampled at 16 kHz. The baseband signal is sampled at 8 kHz and has a bandwidth of 0-4 kHz. Theenhancement band signal is sampled at 8 kHz and has a bandwidth of 4-8kHz.

[0060] According to the present invention, if a user selects only a baseband signal, the base band signal is compatible with a currentlyprovided coding method. Thus, a new system is unnecessary. If anenhancement band signal is also used, speech communication of highquality is possible. Also, speech signals are multiplexed based on thekinds of provided networks. Thus, the speech signals can be efficientlytransmitted. Moreover, echoes are cancelled from the speech signals toallow the use of a loud speaker, giving high quality live sound. Thus,better communication environment can be provided.

[0061] Also, according to the present invention, it is possible torealize computer-readable codes in computer-readable recording media.The computer-readable recording media include all kinds of recordingdevices storing computer-readable data. In other words, thecomputer-readable media include ROM, RAM, CD-ROM, magnetic tapes, floppydisks, optical data storing devices, and the like. Also, it is possiblerealize the computer-readable codes in the form of carrier waves, e.g.,the transmission via the Internet. Moreover, the computer-readablerecording media can be dispersed to computer systems connected to eachother via networks, stored as computer-readable codes and executed.

[0062] While this invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. Therefore, disclosedembodiments must be considered in view of description not limitation.The scope of the present invention is shown in claims not in thedescription and all differences in the range equal to the scope of thepresent invention are construed as being included in the presentinvention.

What is claimed is:
 1. A speech signal transmitter comprising: aquadrature mirror filter for receiving a speech signal, dividing thespeech signal into an enhancement band speech signal and a base bandspeech signal, and outputting the enhancement band speech signal and thebase band speech signal; a base sub-band encoder for receiving andencoding the base band speech signal; an enhancement sub-band encoderfor receiving and encoding the enhancement band speech signal; and anetwork connector for multiplexing the encoded enhancement band speechsignal and the encoded base band speech signal based on the kinds ofnetworks over which speech signals are transmitted, and transmitting themultiplexed signals to the networks.
 2. The speech signal transmitter ofclaim 1, wherein the base sub-band encoder encodes the base band speechsignal based on G.723.1 or G.729A.
 3. The speech signal transmitter ofclaim 1, wherein the network connector simultaneously packets theenhancement band speech signal and the base band speech signal orpackets only the base band speech signal, if the networks are composedonly of networks guaranteeing or of networks not guaranteeing quality ofservice, and packets and transmits the base band speech signal to thenetworks guaranteeing quality of service and packets and transmits theenhancement band speech signal to the networks not guaranteeing qualityof service, if the networks are composed of networks guaranteeingquality of service and networks not guaranteeing quality of service. 4.The speech signal transmitter of claim 1, wherein the speech signal hasa sampling frequency of 16 kHz and a bandwidth of 8 kHz, the enhancementband speech signal has a sampling frequency of 8 kHz and a bandwidth of4 kHz-8 kHz, and the base band speech signal has a sampling frequency of8 kHz and a bandwidth of 0-4 kHz.
 5. The speech signal transmitter ofclaim 1, further comprising an acoustic echo canceller for receiving thespeech signal, canceling echoes from the speech signal, and outputtingthe speech signal to the quadrature mirror filter.
 6. A speech signalreceiver comprising: a network connector for receiving a speech signal,within which an enhancement band speech signal and a base band speechsignal are multiplexed based on the kinds of networks through whichspeech signals are transmitted, demultiplexing the multiplexed speechsignal, and extracting an enhancement band digital speech signal and abase band digital speech signal. an enhancement sub-band decoder forreceiving and decoding the enhancement band speech signal; a basesub-band decoder for receiving and decoding the base band speech signal;and a quadrature mirror filter for synthesizing the decoded enhancementband speech signal and the decoded base band speech signal into a singlespeech signal.
 7. The speech signal receiver of claim 6, wherein thebase sub-band decoder decodes the base band speech signal based onG.723.1 or G.729A.
 8. The speech signal receiver of claim 6, wherein theenhancement band speech signal has a sampling frequency of 8 kHz and abandwidth of 4-8 kHz, and the base band speech signal has a samplingfrequency of 8 kHz and a bandwidth of 0-4 kHz.
 9. The speech signalreceiver of claim 6, further comprising an acoustic echo canceller forcanceling echoes from the synthesized single speech signal.
 10. A methodof transmitting a speech signal comprising: (a) receiving an analogspeech signal and converting the analog speech signal into a digitalspeech signal; (b) dividing the digital speech signal into anenhancement band speech signal and a base band speech signal; (c)encoding the enhancement band speech signal and the base band speechsignal, separately; and (d) multiplexing the encoded enhancement bandspeech signal and the encoded base band speech signal, based on thekinds of networks to which the speech signal is transmitted, andtransmitting the multiplexed enhancement band speech signal and themultiplexed base band speech signal to the networks.
 11. The method ofclaim 10, wherein step (a) comprises canceling echoes from the digitalspeech signal.
 12. The method of claim 10, wherein in step (d), if thenetworks are composed only of networks guaranteeing or of networks notguaranteeing quality of service, then the enhancement band speech signaland the base band speech signal are simultaneously packeted or only thebase band speech signal is packeted, and if the networks are composed ofnetworks guaranteeing quality of service and networks not guaranteeingquality of service, the base band speech signal is packeted andtransmitted to the networks guaranteeing quality of service and theenhancement band speech signal is packeted and transmitted to thenetworks not guaranteeing quality of service.
 13. A method of receivinga speech signal comprising: (a) receiving a speech signal, within whichan enhancement band speech signal and a base band speech signal aremultiplexed based on the kinds of networks through which speech signalsare transmitted; (b) demultiplexing the multiplexed enhancement bandspeech signal and the multiplexed base band speech signal, andextracting an enhancement band digital speech signal and a base banddigital speech signal; (c) decoding the enhancement band digital speechsignal and the base band digital speech signal; and (d) synthesizing thedecoded enhancement band digital speech signal and the decoded base banddigital speech signal into a single speech signal.
 14. The method ofclaim 13, wherein step (d) comprises canceling echoes from thesynthesized one speech signal.
 15. A computer-readable recording mediumonto which is recorded a program for executing a method of any one ofclaims 10 through 14 in a computer.